We have described our approach to modeling the
earthquake-induced ground motion in large, heterogeneous basins on
parallel computers. By paying careful attention to the impact on
parallel execution of all components of the code, we are able to
obtain excellent performance on highly unstructured mesh problems. In
particular, through the use of (i) space- and time-localized absorbing
boundaries; (ii) seismic input in the form of effective boundary or
interior forces applied at the element level; (iii) explicit numerical
techniques for the wave propagation problem; (iv) strict control of
mesh resolution and aspect ratio; and (v) an asymptotically optimal
mesh partitioner, we obtain excellent scalability of the parallel
code. The Archimedes toolset integrates the basic components necessary
for solving general PDE problems involving static unstructured meshes
on parallel distributed memory systems. These components include
meshing, partitioning, and parallel code generation.

We currently solve the meshing, partitioning, and parceling problems
sequentially on a large shared-memory machine. Our ultimate target
problem--the Greater Los Angeles Basin with an excitation of 2 Hz and
with soil deposits having shear wave velocities as low as 200
m/s--will require meshes on the order of hundreds of millions of
elements. Despite the fact that our sequential meshing and
partitioning codes are fast, we may have to parallelize these steps in
order to solve the target problem, primarily for memory reasons. The
scalability of the parallel portion of our code suggests that our
target problem is within reach.